#define PANEL_WINDOW_WIDTH_X_MASK                     0xfff
 
 #define PANEL_WINDOW_HEIGHT                           0x080018
-#define PANEL_WINDOW_HEIGHT_HEIGHT                    27:16
-#define PANEL_WINDOW_HEIGHT_Y                         11:0
+#define PANEL_WINDOW_HEIGHT_HEIGHT_SHIFT              16
+#define PANEL_WINDOW_HEIGHT_HEIGHT_MASK               (0xfff << 16)
+#define PANEL_WINDOW_HEIGHT_Y_MASK                    0xfff
 
 #define PANEL_PLANE_TL                                0x08001C
 #define PANEL_PLANE_TL_TOP                            26:16
 
                reg |= (var->xoffset & PANEL_WINDOW_WIDTH_X_MASK);
                POKE32(PANEL_WINDOW_WIDTH, reg);
 
-               POKE32(PANEL_WINDOW_HEIGHT,
-                       FIELD_VALUE(0, PANEL_WINDOW_HEIGHT, HEIGHT, var->yres_virtual - 1)|
-                       FIELD_VALUE(0, PANEL_WINDOW_HEIGHT, Y, var->yoffset));
+               reg = ((var->yres_virtual - 1) <<
+                      PANEL_WINDOW_HEIGHT_HEIGHT_SHIFT);
+               reg &= PANEL_WINDOW_HEIGHT_HEIGHT_MASK;
+               reg |= (var->yoffset & PANEL_WINDOW_HEIGHT_Y_MASK);
+               POKE32(PANEL_WINDOW_HEIGHT, reg);
 
                POKE32(PANEL_PLANE_TL, 0);